Project summary. Mucolipidosis type IV (MLIV) is a lysosomal storage disorder caused by mutations in the gene coding for a lysosomal ion channel Mucolipin 1 (TRPML1). MLIV causes severe developmental delays, psychomotor retardation, cognitive disabilities, corneal cloudiness and degeneration of the retina. At the cellular level, accumulation of phospholipids and gangliosides is detected, and virtually all patients' cells contain enfolded membranous structures of unknown origin. Since TRPML1 localizes in lysosomes and its downregulation causes severe lysosomal storage phenotype, TRPML1 was suggested to regulate lysosomal function. At present, there is no complete model that explains TRPML1 function in the lysosomes. TRPML1 was suggested to directly regulate intracellular membrane traffic by modulating fusion and/or fission of the organelles within the lower portions of the endocytic pathway. An alternative model suggests that TRPML1 affects lysosomal hydrolysis by regulating lysosomal acidification. The present project aims to directly answer the two main question in MLIV pathogenesis: whether TRPML1 directly regulates membrane traffic in the endocytic pathway and which aspect of TRPML1 permeability makes it indispensable for the proper function of the endocytic pathway. This will be accomplished by a combination of molecular, biochemical and physiological techniques. Specifically, an siRNA driven TRPML1 downregulation system will be used to establish the direct and immediate effects of TRPML1 downregulation on membrane traffic and lysosomal lipid hydrolysis. A structure function analysis of ion permeation through TRPML1 and its close relative TRPML3 will be performed to identify the structural determinants of the unique aspects of TRPML1 ion permeation. The ion permeation through TRPML1 will be specifically modulated in order to establish the aspects of TRPML1 permeability that make TRPML1 indispensable for the proper function of the endocytic pathway. The experiments planned in this proposal will establish the direction for the search for MLIV treatments, identify the new roles for intracellular ion channels in regulation of membrane traffic, lysosomal metabolism and organellar ion homeostasis and provide novel structural information regarding the basic mechanisms of ion channel permeation, rectification and block.